Amplitude-limiting amplifier



Sept. 24, 1940. w. BENZ 2,215,717

AMPLITUDE-LIMITING AMPLIFIER Filed Dec. 15, 1937 Fig.2

OUTPUT 9| INPUT g OUTPUT INVENTOR WERNER BENZ ATTO R N EY Patented Sept. 24, 1940 PATENT OFFICE 2,215,777 AMPLITUDE-LIIVIITING AlrIPLIFIER Werner Benz, Berlin-Tempelhof, Germany, as-

signor to General Elect .tion of New York ric Company, a corpora- Application December '15, 1937, Serial No. 179,880

' In Germany December 16, 19,36

6 Claims.

The problem of amplitude limitation, that is to say, the limitation imposed upon large amplitudes and the suppression of small amplitudes, has been playing an important part in signal and communication arts for many years. The imposition of a limitation upon large amplitudes has the purpose to preclude overloading of amplifier means and undue saturation of the iron of transformers and coils because of the distortions connected therewith. Moreover, undesirable crosstalk in neighboring lines and circuits is to be precluded, mainly with a view to insuring secrecy. The suppression of feeble amplitudes also is intended to decrease cross-talk; but its further aim is to diminish disturbing noises of all sorts such as occasioned by atmospherics or power-current interference or other causes inherent in the circuit organizations themselves such as tube noises or shot or microphonic noises.

Now, arrangements have already been disclosed in the prior art which serve the object of amplitude limitation. In their most rudimentary and simple form, amplitude limiters or amplitude suppressors (amplitude filters) are non-linear. structures or systems when are inserted in series with the line or in shunt thereto, such as glow-tubes, valves, copperor oxide-type rectifiers, etc. Also tubes or valves with ,a pronounced saturation current have been used to act as limiter means, these being so dimensioned that the highest permissible amplitudes will cause the plate current to swing all the way from zero to saturation (under ratedgfull-load conditions). Owing to saturation, a limitation is imposed and .efiected whenever the amplitudes happen to be larger. Where amplifier tubes are employed, a push-pull circuit scheme has become most customary and most widely used in which, by cutting high resistances in the grid circuits the grid potential is practically stabilized upon the arising of grid current initiated as a result of unduly large amplitudes. Where individual or single amplifier tubes are used a limitation of the amplitude, according to certain earlier schemes known in the art, is secured by variation of the grid potential or of the plate D. C. potential as a function of the amplitude.

Now, the present invention is concerned with an amplifier circuit organization in which a negative feedback is used for the object of amplitude limitation. Negative feedback or degeneration has been known in the prior art as a circuit step and means adapted to stabilize the gain (avc)' and for the reduction of non-linear distortions and disturbing actions in general. By

inter-posing an amplitude-dependent resistance in the feedback path, amplitude limitation or suppression is insured. Amplitude-dependent resistance according to the sense of this invention may consist, for instance, of a parallel arrangement of rectifiers in which the direction of transmission is opposite. The resistance of this rectifier scheme in accordance with the characteristic decreases from a crest value, at zero voltage, as the voltage increases. If, then, the resistance is connected in series with the feedback means (transformer coil or resistance), at low amplitudes the resistance is so high that degeneration is practically absent, with the consequence that the ,feeble amplitudes remain undiminished. As the amplitudes grow, the resistance decreases more and more, the negative feedback becomes stronger and causes a diminution and limitation of the amplitudes. ,But if the amplitude-dependent resistance is connected in parallel relation to the feedback element, practically complete action of the negative feedback will take place inversely in the presence of small amplitudes. In the presence of larger amplitudes, on the contrary, the amplitudedependent resistance forms an effective shunt to the feedback means and weakens the eifect of the negative feedback. In other words, suppression of the feeble amplitudes is efiected.

In order to regulate the limiting amplitude value at which the limiting action or suppression is to start, the rectifiers are suitably worked with a convenient biasing potential.

The amplitude limiters, as a general rule, are networks containing amplitude-dependent resistances. In the form of networks they all involve a certain amount of attenuation. which must be neutralized by means of separate or special amplifiers. Contradistinct to the prior art, by combination into a single (unit) circuit organization, the present invention insures great simplicity and by the use of a feedback particularly pronounced limiting action.

In Figs. 1 to 5, for instance, exemplified em.- bodiments of the basic idea of the invention are shown, Figs. 1 and 2 serving for the limitation of large amplitudes, while the arrangements shown in Figs. 3 to 5 are intended to suppress feeble amplitudes.

Referring to Fig. 1, it will be seen that the winding ii in the input transformer U61 through a condenser C, a resistance R and a parallelconnection scheme of rectifiers P is fed with a small portion of the plate alternating potential so that it counteracts the input (signal) alternating potential. In other words, degeneration or a negative feedback is obtained. Where the amplitudes are small, owing to the high resistance whch P represents in this instance, practically no feedback action will arise. In fact, it will only be from a definite magnitude (which may be fixed and determined by suitable adjust ment of the biasing voltage on battery V) that the resistance of P decreases'rapidly and there happens a marked feedback effect which will weaken and limit the amplitudes arising at the grid and thus also at the output transformer U62. The condenser C serves the purpose to preclude D. 0.; resistance It serves for phase and voltage regulation of the feedback path. By the aid of condenser C1, the biasing voltage battery V for the alternating potential is short-circuited.

In the case of Fig. 2, the feedback path at the plate end is associated through a distinct winding f0 in the output transformer U62 rather than directly as in Fig. 1. This makes the provision of the condenser C dispensable, and, by a certain choice of the ratio of transformation between f0 and the primary winding in the output circuit, also the resistance B may be dispensed with.

' As to the rest, the limiting action of the feedback will be initiated just as in Fig. 1 when certain amplitude values have been attained.

In Figs. 3 and 4 the parallel-connection scheme of the rectifiers P is disposed in parallel relation to the feedback winding fi in the transformer U61 rather than in series, as was the case in Figs. 1 and 2. In the presence of small amplitudes, P represents such a high shunt resistance that winding fi remains practically unaffected thereby, with the result that the negative feedback may have its full effect, the small amplitudes being suppressed as a consequence. However, as the amplitudes waX, as a result of the marked decrease of the resistance of P, the shunt action makes itself increasingly more manifest, indeed, it eventually results in a short-circuit so that the negative feedback remains ineffective and inoperative.

However, by using the fact that grid and plate potential present a relative phase shift of degrees, negative feedback could be secured also through a resistance in the cathode or filament circuit in a way as illustrated in Fig. 5. In shunt relation to the cathode resistance Re which acts as a voltage divider in which, therefore, grid and plate alternating potential counteract, there is included, for instance, again a rectifier scheme P comprising paralleled rectifiers operated with a biasing potential. When the amplitudes are low, P represents a high shunt resistance by which RC remains practically unaffected so that the negative feedback may take full effect, while the alternating potential between grid and filament is so reduced that it amounts, for all practical purposes, to a suppression of the small amplitudes. When the amplitudes are high, however, P constitutes a low resistance and thus an effective shunt for R11. The alternating potential between grid and cathode as a result grows; moreover, the counteracting alternating plate voltage becomes lower and hence the negative feedback smaller and weaker so that the gain grows. The condenser C1 serves to preclude D. C., while condenser C2 serves to bridge or shunt the source of plate voltage supply.

It will be understood that the invention is by no means restricted to the use of rectifiers as illustrated in the embodiments shown in Figs. 1 to 5, indeed, it is obvious that the amplitudedepend-ent resistances could consist of any other nonlinear means and structures such as glowtubes presenting a marked saturation current, valves, duodiodes (full-wave rectifiers) etc.

What I claim is:

1. An amplitude-limiting amplifier circuit comprising a vacuum tube provided with input and output electrodes, an input circuit connected to the input electrodes and an output circuit connected to the output electrodes, and a negative feed back path connected between said output and input circuits, said path including a pair of contact rectifiers arranged in parallel opposed relation.

2. An amplitude-limiting amplifier circuit ac cording to claim 1 wherein said negative feed back path includes in addition a series connection of a condenser and a resistance.

3. An amplitude-limiting amplifier circuit according to claim 1 wherein the negative feed back path includes in addition the series connection of a condenser and a resistance, and a coilwhich is inductively coupled to the input circuit.

4. An amplitude-limiting amplifier circuit comprising a vacuum tube provided with input and output electrodes, an input circuit connected to the input electrodes and an output circuit connected to the output electrodes, and a negative feed back path connected between said output and input circuits, said path including a coil inductively coupled to the input circuit and a pair of rectifiers arranged in parallel opposite relation.

5. An amplitude-limiting amplifier according to claim 4 wherein the coil and the pair of rectifiers are connected in series for obtaining large amplitude limitation.

6. An amplitude-limiting amplifier circuit according to claim 4 wherein the coil and the pair of rectifiers are connected in parallel for insuring the suppression of small amplitudes.

WERNER BENZ. 

